This invention relates to processing a hydrocarbon fuel to produce hydrogen for a fuel cell and, more particularly, to a method and system for maintaining temperature control during fuel processing.
Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution of the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation, assignee of the present invention, and having as inventors Swathirajan et al. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes, are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereof.
For vehicular applications, it is desirable to use a liquid fuel, preferably a hydrocarbon or alcohol, such as methanol (MeOH), or gasoline as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to this reaction: CH3OH+H2Oxe2x86x92CO2+3H2. The reforming reaction is an endothermic reaction, which means it requires external heat for the reaction to occur.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in the name of William Pettit in November, 1997, and U.S. Ser. No. 09/187,125, Glenn W. Skala et al., filed Nov. 5, 1998, and each assigned to General Motors Corporation, assignee of the present invention. In U.S. Pat. No., 4,650,722, issued Mar. 17, 1987, Vanderborgh et al. describe a fuel processor comprising a catalyst chamber encompassed by combustion chamber which is in indirect heat transfer relationship with organic fuel which is heated thereby and reacted in the catalyst chamber.
The reaction in the fuel processor (reformer) must be carried out under controlled temperatures to preserve the integrity of the catalyst in the catalytic chamber. The catalyst chamber temperature must be low enough to prevent catalyst degradation, yet high enough to supply the quantities of fuel required by the fuel cell at high load demand.
The present invention is directed to an improved method and system to protect the integrity of catalyst material in a catalytic reaction chamber which constitutes a part of a fuel processor. The method of the invention is useable in a fuel processor which comprises one or more catalytic chambers and a heater housed in a common housing. Gases react in the reactors to form a product suitable for use in a fuel cell. The gases circulate in a stream through the heater and through one or more reactors in the common housing. Each catalytic reactor is downstream of the heater. The temperature of the gas stream is monitored in a location in the housing which is near the heater outlet. Preferably, the heater and catalytic reactor are positioned such that the catalytic reactor is downstream of the heater with the discharge end of the heater in fluid flow communication with an inlet of the catalytic reactor. Preferably, a portion of the gas stream recirculates in the housing in heat transfer relationship with one or more of the reactors. The fuel processor also includes means to inject hydrocarbon fuel into the stream of the recirculating gases. A fan or other means are provided to recirculate gases throughout the catalytic reactor and heater (heat exchanger).
The temperature is monitored between the discharge end of the heater and the inlet of a catalytic reactor, and the monitored temperature is compared to one or more preselected values. In one embodiment, the method of the invention monitors one or more of the following conditions: a relatively low temperature value of the gas stream; a relatively high temperature value of the gas stream; and a rate-of-change of monitored temperature. A relatively low monitored temperature condition indicates that the temperature within one or more of the catalytic chambers of the fuel processor is not adequate to provide the desired quality of hydrogen-containing gas for the fuel cell. A relatively high monitored temperature indicates temperature is possibly damaging to catalytic beds within the fuel processor. A rate-of-change of temperature is useful to indicate that an unacceptable temperature may soon be attained unless corrective action is taken to prevent it.
In a preferred embodiment, the rate of temperature change is monitored to prevent the occurrence of an unacceptably high or low temperature condition. Here, at least two temperatures of the recirculating gas stream are monitored over a period of time. The rate-of-change of temperature versus time is determined. Then the monitored rate-of-change of temperature is compared to a preselected rate-of-change value. The monitoring of rate-of-change of temperature provides proactive means for preventing the occurrence of an unacceptably high temperature in the catalytic reactor.
The method of the invention essentially, indirectly provides an indication of significant failure, such as cracked welds, leaks, and catalyst bed problems, by its measurement of temperature, and rate-of-change of temperature. The placement of the temperature monitoring device in the gas stream between the outlet of the heat exchanger and the inlet of a reactor effectively protects the catalyst beds which are very temperature sensitive. Therefore, it is a very cost effective approach to diagnosing overall system problems while also protecting key system components.
Advantageously, the present monitoring method is adaptable to, and easily implemented in, existing fuel cell systems which comprise a fuel processor. The present method can be implemented in existing process controllers. In addition, the present monitoring method is useable with a variety of fuel cell system fuel processors.